Determining angles of arrival using multipaths

ABSTRACT

In one aspect, a method to determine multipath angles of arrival includes performing an autocorrelation on a first signal received at a first received beam from a signal source, performing a cross-correlation between the first signal and a second signal received at a second receive beam from the signal source, and determining an angle of arrival for a first path from the signal source and an angle of arrival for a second path from the signal source based on the autocorrelation and the cross-correlation.

BACKGROUND

It is known that an underwater vessel (i.e., a submarine) generatessound, which is generally referred to as passive sound, as it travelsthrough the water. The passive sound is generated by a variety ofsources, including, but not limited to, sound generated by a submarinepropulsion system, sound generated by a submarine propeller, and soundgenerated by a submarine electrical power generator. It is known thatsubmarine designers attempt to reduce these and other passive soundsources in order to make a submarine difficult to detect by acousticmeans, therefore remaining as covert as possible.

Some anti-submarine warfare (ASW) sonar systems attempt to detect thepassive underwater sound generated by an enemy submarine. Some other ASWsonar systems attempt to both detect the passive sound and also tolocalize and/or track the enemy submarine. Localization is used toidentify a position of the enemy submarine in azimuth, and/or in range,and/or in depth.

Passive ASW sonar systems attempt to detect, localize, and/or track theenemy submarine using the received passive sound only. The passive sonarsystem can remain covert and undetectable by the enemy submarine. Someknown passive sonar systems use beam-forming techniques to generatereceive beams. The receive beams can be steered azimuthally to detect,localize, and/or track the enemy submarine in azimuth. The receive beamcan also be steered to vertical angles.

Even at relatively short ranges, localization in depth and range is notgenerally possible when receiving only passive sound and depending upona pointing direction of receive beams (directed to a vertical beam steerangle). This is because for any receive beam and associated verticalbeam that points toward an enemy submarine, the enemy submarine can bepositioned at an essentially infinite number of depths and ranges alongthe vertical beam steer angle.

At longer ranges, localization of the enemy submarine in range and depthis made even more difficult by a variety of factors, including but notlimited to, a tendency of the passive sound generated by the enemysubmarine to bend (i.e. refract), primarily in a vertical direction, asthe sound propagates through the water. Therefore, the vertical angle ofarrival at which the greatest amount of sound arrives at the sonarsystem, which is related to a particular receive vertical beam angle,does not necessarily point in the direction of the enemy submarine.

However, it has been shown that vertical angles of arrival may be usedto determine range and depth. Conventional techniques to determinevertical angles of arrival are generally applied to single path verticalangles of arrival at an array. With a single path, the vertical angle ofarrival may be determined using a variety of techniques includingmonopulse or multibeam interpolation techniques.

SUMMARY

In one aspect, a method to determine multipath angles of arrivalincludes performing an autocorrelation on a first signal received at afirst received beam from a signal source, performing a cross-correlationbetween the first signal and a second signal received at a secondreceive beam from the signal source, and determining an angle of arrivalfor a first path from the signal source and an angle of arrival for asecond path from the signal source based on the autocorrelation and thecross-correlation.

In another aspect, an article includes a machine-readable medium thatstores instructions to determine multipath angles of arrival. Theinstructions cause a machine to perform an autocorrelation on a firstsignal received at a first received beam from a signal source, perform across-correlation between the first signal and a second signal receivedat a second receive beam from the signal source; and determine an angleof arrival for a first path from the signal source and an angle ofarrival for a second path from the signal source based on theautocorrelation and the cross-correlation.

In a further aspect, an apparatus to determine multipath angles ofarrival, includes circuitry to perform an autocorrelation on a firstsignal received at a first received beam from a signal source, perform across-correlation between the first signal and a second signal receivedat a second receive beam from the signal source and determine an angleof arrival for a first path from the signal source and an angle ofarrival for a second path from the signal source based on theautocorrelation and the cross-correlation.

In a still further aspect, an acoustic system includes an acousticsensor configured to receive a first signal received at a first receivedbeam from a signal source and a second signal received at a secondreceive beam from the signal source. The acoustic system also includes aprocessing system configured to determine angles of arrival and coupledto the acoustic sensor. The processing system includes anautocorrelation module configured to perform an autocorrelation on thefirst signal, a cross-correlation module configured to perform across-correlation between the first signal and a second signal receivedat a second receive beam from the signal source and an angle processingmodule configured to determine an angle of arrival for a first path fromthe signal source and an angle of arrival for a second path from thesignal source based on the autocorrelation and the cross-correlation.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an acoustic system.

FIG. 2 is a flowchart of a process to determine angles of arrival.

FIG. 3 is a graph of an example of a first receive beam and a secondreceive beam.

FIG. 4 is a graph of an autocorrelation of a first signal received bythe first receive beam in FIG. 3.

FIG. 5 is a graph of a cross correlation of the first signal received bythe first beam and a second signal received by the second receive beamin FIG. 3.

FIG. 6 is a graph of voltage ratios of the first receive beam and thesecond receive beam versus angle.

FIG. 7 is a block diagram of an example of a processing system on whichthe process of FIG. 2 may be implemented.

FIG. 8 is a block diagram of an example of instructions for angle ofarrival determining.

DETAILED DESCRIPTION

Angles of arrival and time delays may be used together to estimate rangeand depth of objects using acoustical methods as shown in U.S. patentapplication Ser. No. 11/422,435, filed on Jun. 6, 2006, titled “Methodsand Systems for Passive Range and Depth Localization,” which isincorporated herein in its entirety and has an obligation of assignmentto the same entity as this patent application. Prior art techniquesdetermine vertical angles by measuring single path arrival angles.

In contrast to prior art approaches, described herein is an approach todetermining angles of arrival (e.g., vertical, horizontal and so forth)in acoustics that uses more than one receive beam for multipaths, e.g.,using two receive beam patterns receiving two signal from two arrivalpaths. However, in using more than one receive beam to receive signalsfrom the multipaths, received signals interact multiplicatively andtherefore it is hard to mathematically separate the received signals.

While the techniques in the description herein focus on broadband andacoustic signals, the techniques may be applied to any broadband signalenvironment.

Referring to FIG. 1, an acoustics system 10 includes a processing system12 and an acoustic sensor system 14 connected to the processing system12 by a network 16. The acoustic sensor system 14 receives acousticsignals from an object 18. For example, the received acoustic signalsmay be from active measures (e.g., a reflected signal resulting from asignal sent from the acoustic system 10) or from passive measures (e.g.,receiving an acoustic signal resulting from movement of the object 18 orother acoustics emanating from the object 18). The received signals maytake more than one path back to the acoustic sensor system 14. Forexample, one path is a first arrival path 22 is reflected off an oceanfloor 30. Another path, a second arrival path 24, is reflected off asurface 40 of water. In other examples, either the first arrival path 22or the second arrival path 24 may be a direct path to the acousticsensor system 14. An angle, Y, represents the angle of arrival of thefirst arrival path 22. An angle, Z, represents the angle of arrival ofthe second arrival path 24. In this example, the angles of arrival, Yand Z, are vertical angles. In other examples, such as a ship receivingsignals from under water, the angles of arrival may be horizontalangles.

In one example, the acoustic sensor system 14 may include one or moresonar sensors, such as sonobuoys. In another example, the acousticsensor system 14 is a sonar sensor located aboard a submarine. In oneexample, the network 16 may be a wired or a wireless network.

The processing system 12 may be located on a ground-based platform(e.g., in a building, in a vehicle and so forth), a space-based platform(e.g., a satellite, a space-vehicle and so forth), a sea-based platform(e.g., a ship, a submarine, a buoy, an anchored sea structure, atorpedo, an undersea robotic vehicle and so forth) or on an air-basedplatform (e.g., an aircraft, a helicopter, a missile and so forth).

In one example, the processing system 12 may be co-located (i.e., on thesame platform) with the acoustic sensor system 14. In other examples,the processing system 12 is not co-located with the acoustic sensorsystem 14.

As will be shown below, the approach described herein uses more than onearrival path to determine the angles of arrival. The following is anillustrative mathematical support to determine angles of arrival usingtwo receive beams to receive two signals from two arrival paths.

For a first arrival path, a relative arrival time is t₁, a relativeamplitude of a medium (e.g., an ocean) is A₁, a first beam patternvoltage amplitude is v₁₁ and a second beam pattern voltage amplitude isv₂₁; and for a second arrival path, a relative arrival time is t₂, arelative amplitude of the medium is A₂, a first beam pattern voltageamplitude is v₁₂ and a second beam pattern voltage amplitude is v₂₂,where A_(i) is the complex medium transfer function of an i-th path,v_(ij) is a j-th beam pattern response for the i-th path arrival angleand t_(i) is the travel time associated with the i-th path. Voltageamplitude, v_(ij), is assumed to be real to simplify the analysis sincethe extension for a complex beam pattern response is straightforward forone of ordinary skill in the art. It is assumed that t₂ is greater thant₁.

If s(t) represents a signal source, a received signal at the firstreceive beam from the signal source received from the first arrival pathand the second arrival path is described as:

s ₁(t)=v ₁ ·A ₁ ·s(t−t ₁)+v ₁₂ ·A ₂ ·s(t−t ₂).

A received signal at the second beam source from the signal sourcereceived from the first arrival and the second arrival path is describedas:

S ₂(9 t)=v ₂₁ ·A ₁ ·s(t−t ₁)+v ₂₂ ·A ₂ ·s(t−t ₂).

The two signal components comprising s₁(t) are separated by τ=t₂−t₁. Theautocorrelation of s₁ results in a correlator output of exhibiting peaksat delays of τ=0, ±(t₂−t₁). The magnitude of the peak at τ=0 is given by

[|v ₁₁ ·A ₁ |+|v ₁₂ ·A ₂|² ]·<s ²>,

where <s²> is the average energy of the source. The magnitude of thepeak at τ=0 provides the measure of the total signal energy, but is notuseful for determining the angles of arrival because the signals fromthe two paths are combined.

Next consider the peak at τ=+(t₂−t₁)=τ₂₁, where the signal is beingdelayed. The magnitude of the peak is given by:

ρ₁₁(τ₂₁)=v ₁₁ ·v ₁₂ ·A ₁ ·A ₂ *<s ²>.

Similarly, if the signal is advanced by τ=−(t₂−t₁), then

ρ₁₁(τ₂₁)=v ₁₁ ·v ₁₂ ·A ₁ ·A ₂ *<s ²>

ρ₁₁(τ₂₁) and ρ₁₁(−τ₂₁) are identical and contain the product of the beampattern at the two different angles, v₁₁·v₁₂, but it is not possible touniquely solve for an angle pair from this product.

Next consider the cross correlation of signals received at receive beams1 and 2, which will produce peaks at the same delays as the aboveautocorrelation because the receive beams are collocated. At τ=+τ₂₁,where the copy of beam 2 signal is being delayed, the magnitude of thepeak is given by

ρ₁₂(τ₂₁)=v ₁₂ ·v ₂₁ ·A ₁ ·A ₂ *<s ²>.

Similarly if beam 2 is advanced by τ=−(t₂−t₁), the magnitude of the peakis given by:

ρ₁₂(−τ₂₁)=v ₂₂ v ₁₁ ·A ₁ ·A ₂ *<s ²>.

Once again, these terms contain the product of two unknown beam patternvalues and it is not possible to uniquely determine the angles ofarrival.

However, using the auto correlation and cross correlation together, onemay solve for the angles of arrival. For example, let the ratio of thecross correlation peak amplitude to the corresponding autocorrelationpeak be denoted by X(τ), then

X(τ₂₁)=(ρ₁₂(τ₂₁))/(ρ₁₁(τ₂₁))

X(τ₂₁)=(v ₁₁ ·v ₂₂ ·A ₁ ·A ₂ *<s ²>)/(v ₁₁ · ₁₂ ·A ₁ ·A ₂ *<s ²>)

X(τ₂₁)=v ₂₂ /v ₁₂.

Since the ratio of the beam pattern main lobes is a monotonic function(i.e., a unique relationship between the beam pattern main lobe ratiosand the angle of arrival over the interval of interest is guaranteed),the ratio will enable one to determine the second path arrival angle byinverting or interpolating the beam pattern ratio function using themeasured value of X(τ).

Similarly, the ratio of the correlation peaks for τ=−τ₂₁ produces theratio for the first path angle of arrival, that is

X(−τ₂₁)=v ₂₁ /v ₁₁.

Referring to FIG. 2, a process 60 is one example to determine angles ofarrival. Process 60 forms a first receive beam (64). For example, thefirst receive beam is formed as a detection beam. In one example, thefirst receive beam 102 has a main lobe centered at −5 degrees in a graph100 (See FIG. 3). Process 60 forms a second receive beam (68). Forexample, the second receive beam is formed as a detection beam thatoverlaps the first receive beam. In one example, the second receive beamhas a main lobe centered at +5 degrees in the graph 100 (See FIG. 3)

For convenience, the present example considers the case of collocatedbeams. The technique will operate with separate arrays provided that thechannel amplitude functions are comparable or can be estimated, and thatthe corresponding multipath pair delays can be matched.

When more than two paths and/or two beams are available, the presenttechnique can be applied to each of the path-pair and/or beamcombination. This process may generate multiple estimates of the samearrival path; in this case, the estimates can be weighted and averagedto obtain a better estimate than that achieved using a single path.

Process 60 performs an autocorrelation of a first signal received at thefirst receive beam (76). In one example, the first arrival path 22 hasrelative travel time, t₁, of 17 ms with a relative amplitude, A₁, ofzero dB. The second arrival path 24 has a relative travel time, t₂, of29 ms and a relative amplitude, A₂, of −2 dB. The τ is t₂−t₁ or 12 ms.The unknown angles to solve are Y and Z. The auto correlation of thefirst signal received at the first receive beam 102 (FIG. 3) is shown ina graph 110 depicted FIG. 4.

Process 60 cross-correlates a second signal received at the secondreceive beam with the first signal (82). In one example, the crosscorrelation of the first signal and the second signal is shown in FIG.5.

Process 60 determines peaks (86). For example the autocorrelation peaksand the cross correlation peaks are determined. In one example of theauto correlation, one peak 112 is at 12 ms corresponding to anautocorrelation amplitude of 0.28 and the other peak 114 is at −12 mscorresponding to an autocorrelation amplitude of 0.28 (see FIG. 4). Thecorrelation peak at time equal zero has been omitted for scalingpurposes. In one example of the cross correlation, one peak 142 is at 12ms corresponding to a cross correlation amplitude of 0.125 and the otherpeak 144 is at −12 ms corresponding to a cross correlation amplitude of0.38 (See FIG. 5). The correlation peak at time equal zero has beenomitted for scaling purposes.

Process 60 forms voltage beam amplitude ratios for the same time delay(92). For example, the voltage beam amplitude ratio of the second pathis given by:

v ₂₂ /v ₁₂=(ρ₁₂(+τ₂₁))/(ρ₁₁(+τ₂₁)),

and the voltage beam amplitude ratio of the first path is given by:

v ₂₁ /v ₁₁=(ρ₁₂(−τ₂₁))/(ρ₁₁(−τ₂₁))

Using the autocorrelation and cross correlation peaks in the example forprocessing block 86:

v ₂₁ /v ₁₁=0.38/0.28=1.35

and

v ₂₂ /v ₁₂=0.125/0.28=0.45

Process 60 determines beam pattern voltage ratio versus angle (94). Forexample in FIG. 6, a graph 160 has a curve 162 of the voltage ratios ofthe first receive beam divided by the second receive beam (from FIG. 3)versus angles.

Process 60 solves for angles of arrival using the voltage beam amplituderatios (96). In the preceding examples for processing block,v₂₂/v₁₂=0.125/0.28=0.45 at a point 172 corresponds to a first patharrival angle (Angle Y) of −5 degrees and v₂₁/v₁₁=0.38/0.28=1.35 at apoint 174 corresponds to a second path arrival angle (Angle Z) of +9degrees.

Referring to FIG. 7, in one example, the processing system 12 may be aprocessing system 12′. The processing system 12′ includes a processor222, a volatile memory 224, a non-volatile memory 226 (e.g., hard disk)and a network transceiver 225. The non-volatile memory 226 storescomputer instructions 232, an operating system 236 and data 238. Thecomputer instructions 232 include instructions to determine an angle ofarrival 234. In one example depicted in FIG. 8, the instructions todetermine an angle of arrival 234 include auto correlation instructions302 (e.g., instructions to perform processing block 76 of FIG. 2),cross-correlation instructions 304 (e.g., instructions to performprocessing block 82 of FIG. 2) and angle processing instructions 306(e.g., instructions to perform processing blocks 86, 92, 94 and 96 ofFIG. 2). The transceiver 225 is used to communicate with the acousticsensor system 14. In one example, the computer instructions 232 areexecuted by the processor 222 out of volatile memory 224 to performprocess 60.

Process 60 is not limited to use with the hardware and software of FIG.7; it may find applicability in any computing or processing environmentand with any type of machine or set of machines that is capable ofrunning a computer program. Process 60 may be implemented in hardware,software, or a combination of the two. Process 60 may be implemented incomputer programs executed on programmable computers/machines that eachincludes a processor, a storage medium or other article of manufacturethat is readable by the processor (including volatile and non-volatilememory and/or storage elements), at least one input device, and one ormore output devices. Program code may be applied to data entered usingan input device to perform process 60 and to generate outputinformation.

The system may be implemented, at least in part, via a computer programproduct, (e.g., in a machine-readable storage device), for execution by,or to control the operation of, data processing apparatus (e.g., aprogrammable processor, a computer, or multiple computers)). Each suchprogram may be implemented in a high level procedural or object-orientedprogramming language to communicate with a computer system. However, theprograms may be implemented in assembly or machine language. Thelanguage may be a compiled or an interpreted language and it may bedeployed in any form, including as a stand-alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program may be deployed to be executed on onecomputer or on multiple computers at one site or distributed acrossmultiple sites and interconnected by a communication network. A computerprogram may be stored on a storage medium or device (e.g., CD-ROM, harddisk, or magnetic diskette) that is readable by a general or specialpurpose programmable computer for configuring and operating the computerwhen the storage medium or device is read by the computer to performprocess 60. Process 60 may also be implemented as a machine-readablestorage medium, configured with a computer program, where uponexecution, instructions in the computer program cause the computer tooperate in accordance with process 60.

The processes described herein are not limited to the specificembodiments described herein. For example, the process 60 is not limitedto the specific processing order of FIG. 2. Rather, any of theprocessing blocks of FIG. 2 may be re-ordered, combined or removed,performed in parallel or in serial, as necessary, to achieve the resultsset forth above.

The processing blocks in FIG. 2 associated with implementing the systemmay be performed by one or more programmable processors executing one ormore computer programs to perform the functions of the system. All orpart of the system may be implemented as, special purpose logiccircuitry (e.g., an FPGA (field programmable gate array) and/or an ASIC(application-specific integrated circuit)).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. Elements of a computer include aprocessor for executing instructions and one or more memory devices forstoring instructions and data.

Elements of different embodiments described herein may be combined toform other embodiments not specifically set forth above. Otherembodiments not specifically described herein are also within the scopeof the following claims.

1. A method to determine multipath angles of arrival comprising:performing an autocorrelation on a first signal received at a firstreceived beam from a signal source; performing a cross-correlationbetween the first signal and a second signal received at a secondreceive beam from the signal source; and determining an angle of arrivalfor a first path from the signal source and an angle of arrival for asecond path from the signal source based on the autocorrelation and thecross-correlation.
 2. The method of claim 1 wherein performing anautocorrelation on a first signal received at a first received beamcomprises performing an autocorrelation on a first signal comprising afirst signal component from the first path and a second signal componentfrom the second path, the first signal component and the second signalcomponent are separated in time.
 3. The method of claim 1, furthercomprising: determining autocorrelation peaks from the autocorrelation;and determining cross-correlation peaks from the cross-correlation. 4.The method of claim 3, further comprising determining voltage beamamplitude ratios for an equivalent time delay using the autocorrelationpeaks and the cross-correlation peaks.
 5. The method of claim 4 whereindetermining an angle of arrival for a first path and an angle of arrivalfor a second path comprises using the voltage beam amplitude ratios. 6.The method of claim 5, further comprising determining a beam patternratio of the first beam and the second beam versus angle to form acurve, wherein using the voltage beam amplitude ratios comprises usingthe voltage beam amplitude ratios to locate the angle of arrival for thefirst path and the angle of arrival for the second path on the curve. 7.The method of claim 1 wherein determining an angle of arrival for afirst path and an angle of arrival for a second path comprisesdetermining vertical angles of arrival.
 8. The method of claim 1 whereindetermining an angle of arrival for a first path and an angle of arrivalfor a second path comprises determining horizontal angles of arrival. 9.The method of claim 1 wherein determining an angle of arrival for afirst path and an angle of arrival for a second path comprisesdetermining angles of arrival for received acoustic signals.
 10. Anarticle comprising a machine-readable medium that stores instructions todetermine multipath angles of arrival, the instructions causing amachine to: perform an autocorrelation on a first signal received at afirst received beam from a signal source; perform a cross-correlationbetween the first signal and a second signal received at a secondreceive beam from the signal source; and determine an angle of arrivalfor a first path from the signal source and an angle of arrival for asecond path from the signal source based on the autocorrelation and thecross-correlation.
 11. The article of claim 10 wherein instructionscausing a machine to perform an autocorrelation on a first signalreceived at a first received beam comprises instructions causing amachine to perform an autocorrelation on a first signal comprising afirst signal component from the first path and a second signal componentfrom the second path, the first signal component and the second signalcomponent are separated in time.
 12. The article of claim 10, furthercomprising instructions causing a machine to: determine autocorrelationpeaks from the autocorrelation; and determining cross-correlation peaksfrom the cross-correlation.
 13. The article of claim 12, furthercomprising instructions causing a machine to determine voltage beamamplitude ratios for an equivalent time delay using the autocorrelationpeaks and the cross-correlation peaks.
 14. The article of claim 13wherein instructions causing a machine to determine an angle of arrivalfor a first path and an angle of arrival for a second path comprisesinstructions causing a machine to use the voltage beam amplitude ratios.15. An apparatus to determine multipath angles of arrival, comprising:circuitry to: perform an autocorrelation on a first signal received at afirst received beam from a signal source; perform a cross-correlationbetween the first signal and a second signal received at a secondreceive beam from the signal source; and determine an angle of arrivalfor a first path from the signal source and an angle of arrival for asecond path from the signal source based on the autocorrelation and thecross-correlation.
 16. The apparatus of claim 15 wherein the circuitrycomprises at least one of a processor, a memory, programmable logic andlogic gates.
 17. The apparatus of claim 15 wherein circuitry to performan autocorrelation on a first signal received at a first received beamcomprises circuitry to perform an autocorrelation on a first signalcomprising a first signal component from the first path and a secondsignal component from the second path, the first signal component andthe second signal component are separated in time.
 18. The apparatus ofclaim 15, further comprising instructions circuitry to: determineautocorrelation peaks from the autocorrelation; and determinecross-correlation peaks from the cross-correlation.
 19. The apparatus ofclaim 18, further comprising circuitry to determine voltage beamamplitude ratios for an equivalent time delay using the autocorrelationpeaks and the cross-correlation peaks.
 20. The apparatus of claim 19wherein circuitry to determine an angle of arrival for a first path andan angle of arrival for a second path from comprises circuitry to usethe voltage beam amplitude ratios.
 21. An acoustic system, comprising:an acoustic sensor configured to receive a first signal received at afirst received beam from a signal source and a second signal received ata second receive beam from the signal source; and a processing systemconfigured to determine angles of arrival and coupled to the acousticsensor, the processing system comprising: an autocorrelation moduleconfigured to perform an autocorrelation on the first signal; across-correlation module configured to perform a cross-correlationbetween the first signal and a second signal received at a secondreceive beam from the signal source; and an angle processing moduleconfigured to determine an angle of arrival for a first path from thesignal source and an angle of arrival for a second path from the signalsource based on the autocorrelation and the cross-correlation.
 22. Theacoustic system of claim 21 wherein the acoustic sensor comprises asonar sensor.
 23. The acoustic system of claim 22 wherein the first pathand the second path include a water medium.
 24. The acoustic system ofclaim 22 wherein the signal source is associated with passive sonarmeasures.